1. Field of the Invention
The present invention relates to a method for calibrating the signals transmitted from an optical disk drive controller to an optical pickup head and a device for the same, and more particularly, to a calibration method and device calibrating the duty cycle distortion of the signal due to different ground voltage references between an optical pickup head and an optical disk drive controller.
2. Description of Prior Arts
Conventional optical disk drive systems consist of two major components, an optical pickup head and an optical disk drive controller. The optical pickup head comprises a laser diode and a laser diode driver for emitting a laser beam to the optical disk and a sensor for receiving a reflected beam from the optical disk. The weight of the optical pickup head is a primary issue for its movability, and thus the optical disk drive controller and the optical pickup head are separated devices in an optical read/write system.
Generally speaking, the optical disk drive controller placed along with other electronic components on a printed circuit board is fixed to the casing of the optical disk drive. The optical pickup head consisting of the laser diode driver is placed in one another printed circuit board, which is movable with respect to the optical disk drive system. The printed circuit board having the optical disk drive controller connects to the printed circuit board having the laser diode driver by a flexible cable for data/signal communication.
FIG. 1 is a functional block diagram showing the optical disk drive controller 10 and the optical pickup head 15. The optical disk drive controller 10 controls operations of the optical disk drive such as the rotation speed, start-up, stop, etc. Signals generated by a control module 16 are transmitted to the laser diode driver 12 through signal channels 18. Herein, a signal channel refers to the route for signal transmission, including the signal transmitting circuitry in the optical disk drive controller 10 (such as the circuitry on the printed circuit board), the flexible cable, and the signal transmitting circuitry in the optical pickup head 15.
FIG. 2 illustrates a conventional laser diode driver 12. The laser diode driver 12 receives three power control signals, three write strategy signals and two high-frequency modulation parameters, and one high-frequency modulation signal (OSCEN) from the optical disk drive controller 10. The three power control signals are the read power control signal (RADJ), the first write power control signal (WADJ1) and the second write power control signal (WADJ2), while the three write strategy signals are RCLK, WCLK1 and WCLK2 and the two high-frequency modulation parameters are FADJ and AADJ. Thereafter, the laser diode driver 12 generates a drive signal (LDOUT) to drive the laser diode 14. The laser diode driver 12 further includes a read channel driving level generator 20, a first write channel driving level generator 22, a second write channel level generator 24, a high-frequency modulator (HFM) 26 and switches 202, 222 and 242. In addition, the laser diode driver 12 may further include a controlling signal (LD_ENABLE) and a switch 282 for enabling/disabling the drive signal (LDOUT).
The read channel driving level generator 20 receives the read power control signal RADJ and outputs a current signal IRADJ after some signal processing such as signal amplification and conversion from voltage to current. The current signal IRADJ goes through the switch 202 and drives the drive signal LDOUT. The first waveform reshaping unit 251 receives the write strategy signal RCLK, reshape the write strategy signal into a rectangular waveform, which in turn controls the on/off state of the switch 202 by the reshaped write strategy signal RCLK.
The first write channel driving level generator 22 receives the first write power control signal WADJ1 through the first write channel and outputs another current signal IWADJ1 after some signal processing such as signal amplification and conversion from voltage to current. The current IWADJ1 passes through the switch 222 and drives to the drive signal LDOUT. The second waveform reshaping unit 252 receives the write strategy signal WCLK1 and reshapes the signal WCLK1 into a rectangular waveform. The reshaped write strategy signal WCLK1 is used to control the on/off state of the switch 222.
The second write channel driving level generator 24 receives the second write power control signal WADJ2 through the second write channel and generates another current signal IWADJ2 after processing, e.g. by signal amplification and conversion from voltage to current, the second write power control signal WDAJ2. The current IWADJ2 passes through the switch 242 and drives to the drive signal LDOUT. The third waveform reshaping unit 253 receives the write strategy signal WCLK2 and reshapes WCLK2 to be a rectangular waveform. The reshaped write strategy signal WCLK2 is used to control the on/off state of the switch 242.
The high-frequency modulator 26 receives the high-frequency parameters FADJ and AADJ for respectively controlling the frequency and amplitude, then generates another current signal IHFM that is a high-frequency signal. The current signal IHFM passes through the switch 262 and drives to the drive signal LDOUT. The switch 262 is controlled by the high-frequency modulation signal OSCEN. In addition, the laser diode driver 12 can further includes a switch 282 controlled by the controlling signal LD_ENABLE to enable or disable the drive signal LDOUT.
Refer to FIGS. 3a to 3e, the timing diagrams illustrating waveforms of signals RCLK, OSCEN, WCLK1, WCLK2 and LDOUT of laser diode driver 12 in FIG. 2. As usual, the horizontal axis represents the time axis, while the vertical axis represents the signal level axis. During the data reading stage 32, the light intensity of the laser beam emitted from the laser diode 14 is relative lower and only the read power control signal RADJ is activated. The signal level of the read strategy signal RCLK is at “high” level to turn on the switch 202, while the write strategy signals WCLK1 and WCLK2 are at “low” level to turn off the switches 222 and 242. In the meantime, the high-frequency modulation signal OSCEN is at “high” level to turn on the switch 262. Consequently, the drive signal LDOUT during the data reading stage 32 is as shown in FIG. 3e. 
During the data writing stage 34, the laser beam intensity is increased to a level higher than that during the data reading stage and the first write power control signal WADJ1 and the second write power control signal WADJ2 will be activated during the data writing stage. The write strategy clock RCLK is at “high” level to turn on the switch 202, while the write strategy signals WCLK1 and WCLK2 will be switching from “high” to “low” or vice versa according to the predetermined write strategy. As a result, switches 222 and 242 are turned on/off accordingly. The high-frequency modulation signal OSCEN is at “low” level to turn off the switch 262. The exemplar drive signal LDOUT at the data writing stage 34 is shown in FIG. 3e. 
Due to the signal transmission characteristics and the different ground voltage references between the optical disk drive controller and the optical pick-up head, the duty cycle of the signal received by the laser diode driver 12 is not identical to that of the transmitted one. And, the higher the signal frequency is the larger the duty cycle distortion will be. As shown as in FIG. 3d, the transmitted write strategy signal is WCLK2, the signal received by the laser diode driver 12 is WCLK2* with duty cycle distorted. Then the drive signal LDOUT* generated by the laser diode driver 12 is distorted, which will degrade the performance of the optical disk drive.
FIG. 4 shows the waveforms 100 of the transmitted signal, the received signal, and the reshaped received signal transmitted from the control module 16, and the waveform 102 of the signal received at the laser diode driver 12, and the waveform 104 of the reshaped received signal. Basically the waveform 100 of the signal transmitted from the control module 16 is pretty close to an ideal rectangular waveform with two voltage values of VH and VL. However, the signal received by the laser diode driver 12 is no longer an ideal rectangular waveform due to the transmission characteristics of the signal channel. FIG. 4 shows that the signal waveform 102 has a transition time TR from the VL to VH and another transition time TF from VH to VL and usually TR is not necessarily equal to TF.
The laser diode driver 12 reshapes the received waveform 102 so as to have the reshaped waveform 104. To reshape a waveform needs a predetermined reference voltage VT which is expected to be the average of VH and VL. Signal voltages higher than the reference voltage VT are reshaped to be VH and signal voltages lower than VT are reshaped to be VL. Obviously, different VT will cause the reshaped waveform to have different duty cycle. As mentioned earlier, optical pickup head 15 and the optical disk drive controller 10 are separated components and thus refer to different ground voltages. As a result, the reference voltage VT adopted by the optical pickup head 15 and the optical disk drive controller 10 are usually different, leading to the duty cycle distortion of the signal that is received and reshaped by the laser diode driver 12.